Author Topic: Do I need to adjust angles for higher refractive index stones?  (Read 326 times)


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Do I need to adjust angles for higher refractive index stones?
« on: September 11, 2018, 06:49:27 AM »
I have a small quantity of Lithium Tantalate (r.i. ~2.2). I was planning on  cutting a Rand's Corner Cut rectangle.
The design's angles I have are for r.i. of 1.54.

How necessary do you think it is to adjust the angles? I can muddle my way through gemcad and wondered if it is likely to make much difference. Some sources appear to indicate that one can simply keep the same angles when cutting in a higher r.i.


Ian Robinson  .. uk


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Re: Do I need to adjust angles for higher refractive index stones?
« Reply #1 on: September 11, 2018, 08:01:20 AM »
I find many designs will optimizes for one set of angles but there are a lot that don't. I often find my best designs will use one set of angles.

So I just put the design(Rands corner cut rectangle) into gemray and I'd say cut as is, any improvements from optimizing are small :) It's a very nice design.

« Last Edit: September 12, 2018, 07:38:59 AM by MakkyBrown »
We are a family of 4 from Southern Tasmania.


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Re: Do I need to adjust angles for higher refractive index stones?
« Reply #2 on: September 13, 2018, 07:35:13 PM »
If the crown of a design is set high or has facets that go from girdle to table in a low-RI design, I find you need to adjust the crown a bit to get a similar pattern.
Voltolini's Sakura design has this issue, renders in CZ lack the headshadow that gives the design it's characteristic petals.
In most cases vastly higher RIs tend to sacrifice pavilion-generated patterns for extra scintillation, but you can fix it easily with a grid rendering tool like gemRayX-equipped BOGRay or GemCutStudio.

If the design doesn't have those features, it shouldn't be a problem, though. Occasionally I find that CZ and higher RIs can cause a reflection pattern to become too chaotic for my tastes, but deepening the pavilion helps.


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Re: Do I need to adjust angles for higher refractive index stones?
« Reply #3 on: September 17, 2018, 11:02:44 PM »
I always run a new design through an optimiser set to the ŔI of my material to check.  Some designs dont need adjusing, with others a tweek can double the crispness of the design.

Higher RI means lower critical angles.  In dark material lowering the pavillion from a design angle of 42 degrees will brighten the stone and make it bright.

It's worth testing using an optimisation routine, or a programme like Gem Cut Studio, which is easier to learn than Gemcad.

Happy faceting
Yours Sincerely
Flash (Gordon)


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